Long-term variability of the coastal ocean stratification in the Gulf of Naples:Two decades of monitoring the marine ecosystem at the LTER-MC site, between land and open Mediterranean sea

We analyze 20 years (2001–2020) of temperature and salinity profiles at the LTER–MC coastal station in the Gulf of Naples, Mediterranean Sea. Surface and bottom layers show weak increases of temperature (+0.01°C ± 0.01°C /year and +0.03°C ± 0.02°C /year, 2005–2019); water-columns budgets (heat, freshwater) show pseudo-periodic oscillations every 3 to 5 years, and weak linear trends. Seasonal minimum of salinity occurs 2 months later than the runoff peak, pointing to the importance of horizontal circulation in regulating the inshore–offshore exchanges and the residence time of freshwater. Inter-annual variations of the mixed layer depth (MLD) indicate a shallowing (-1.27m ± 0.38m /year during winter) and a shortened time span of the fully mixed water-column. A visible decadal shift in the external forcings suggests an influence of winterly wind stress in 2010–2019, that prevailed over dominant buoyancy fluxes in 2001–2009. Changes are visible in the large-scale indices of the North Atlantic and Western Mediterranean Oscillations and highlight the role of wind direction, offshore or inshore oriented, in disrupting the stratification driven by freshwater runoff. A Random Forest Regression confirms that role and quantifies the MLD's drivers importance. This allows for a reliable prediction of the stratification using external variables independent from the in-situ observations.